Archiv der Mathematik

A reﬁnement of the simple connectivity at inﬁnity of groups By Louis Funar and Daniele Ettore Otera

Abstract. We give another proof for a result of Brick ([2]) stating that the simple connectivity at inﬁnity is a geometric property of ﬁnitely presented groups. This allows us to deﬁne the rate of vanishing of π1∞ for those groups which are simply connected at inﬁnity. Further we show that this rate is linear for cocompact lattices in nilpotent and semi-simple Lie groups, and in particular for fundamental groups of geometric 3-manifolds.

1. Introduction. The ﬁrst aim of this note is to prove the quasi-isometry invariance of the simple connectivity at inﬁnity for groups, in contrast with the case of spaces. We recall that: D e f i n i t i o n 1. The metric spaces (X, dX ) and (Y, dY ) are quasi-isometric if there are constants λ, C and maps f : X→Y , g : Y →X (called (λ, C)-quasi-isometries) such that the following: dY (f (x1 ), f (x2 )) λdX (x1 , x2 ) + C, dX (g(y1 ), g(y2 )) λdY (y1 , y2 ) + C, dX (f g(x), x) C, dY (gf (y), y) C, hold true for all x, x1 , x2 ∈ X, y, y1 , y2 ∈ Y . D e f i n i t i o n 2. A connected, locally compact, topological space X with π1 X = 0 is simply connected at inﬁnity (abbreviated s.c.i. and one writes also π1∞ X = 0) if for each compact k X there exists a larger compact k K X such that any closed loop in X − K is null homotopic in X − k. Mathematics Subject Classiﬁcation (2000): 20F32, 57M50. Partially supported by GNSAGA. S4654

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Louis Funar and Daniele Ettore Otera

arch. math.

R e m a r k 1. The simple connectivity at inﬁnity is not a quasi-isometry invariant of spaces ([15]). In fact (S 1 × R) ∪ D 2 and (S 1 × R) ∪ D 2 are simply connected, S 1 ×Z

S 1 ×{0}

quasi-isometric spaces although the ﬁrst is simply connected at inﬁnity while the second is not. This notion extends to a group-theoretical framework as follows (see [3], p. 216): D e f i n i t i o n 3. A group G is simply connected at inﬁnity if for some (equivalently any) ﬁnite complex X such that π1 X = G one has π1∞ X˜ = 0, where X˜ denotes the universal covering of X. The independence on the particular complex X is proved in [17] and [15]. Roughly speaking the simple connectivity at inﬁnity depends only on the 2-skeleton and any ﬁnite 2-complex corresponds to a presentation of G. Since Tietze transformations act transitively on the set of group presentations it sufﬁces to check the invariance under such moves. We refer to [17] for details. All groups considered in the sequel will be ﬁnitely generated and a system of generators determines a word metric on the group. Although this depends on the chosen generating set the different word metrics are quasi-isometric. Therefore properties which are invariant under quasi-isometries are independent on the particular word metric and will be called geometric properties. It is well-known that being ﬁnitely presented, word hyperbolic or virtually free are geometric properties, while being virtually solvable or virtually torsionfree are not geometric (see [5]). Our main result is : Theorem 1 ([2]). The simple connectivity at inﬁnity of groups is a geometric property. This was originally proved by Brick in [2]. We provide a simpler and more conceptual proof, by analyzing the colored Rips complex. R e m a r k 2. It seems that the fundamental group at inﬁnity, whenever it is well-deﬁned (see [9] for an extensive discussion on this topic), should also be a quasi-isometry invariant of the group. D e f i n i t i o n 4. Let X be a simply connected non-compact metric space with π1∞ X = 0. The rate of vanishing of π1∞ , denoted VX (r), is the inﬁmal N (r) with the property that any loop which sits outside the ball B(N (r)) of radius N (r) bounds a 2-disk outside B(r). R e m a r k 3. It is easy to see that VX can be an arbitrary large function. It is customary to introduce the following equivalence relation on functions: f ∼ g if there exists constants ci , Cj (with c1 , c2 > 0) such that c1 f (c2 R) + c3 g(R) C1 f (C2 R) + C3 . It is an easy consequence of the proof of theorem 1 that the equivalence class of VX (r) is a quasi-isometry invariant. In particular VG = VX is a quasi-isometry invariant of the group G

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G, where X G is the universal covering space of a compact simplicial complex XG , with π1 (XG ) = G and π1∞ (G) = 0. R e m a r k 4. For most groups G coming from geometry VG is trivial, i.e. linear. Obviously if M has an Euclidean structure then Vπ1 (M) is linear. Since metric balls in the hyperbolic space are diffeomorphic to standard balls in Rn one derives that Vπ1 (M) is linear for any compact hyperbolic manifold M. R e m a r k 5. Notice that there exists (see [4]) word hyperbolic groups G (necessary of dimension n 4 by [1]) which are not simply connected at inﬁnity and hence VG is not deﬁned. Moreover if G is a word hyperbolic torsion-free group with π1∞ (G) = 0 then it seems that VG is linear. Theorem 2. VG is linear for uniform lattices in: (1) semi-simple Lie groups. (2) nilpotent groups. (3) solvable stabilizers of horospheres in product of symmetric spaces of rank at least two, or generic horospheres in products of rank one symmetric spaces. Interesting examples of groups for which π1∞ (G) = 0 are the (inﬁnite) fundamental groups of geometric 3-manifolds (and conjecturally of all 3-manifolds). We can show that: Corollary 1. The fundamental groups of geometric 3-manifolds have linear VG . R e m a r k 6. The existence of groups G acting freely and cocompactly on Rn , which have super-linear VG seems most likely. The examples described in ([11], Section 4), which have large acyclicity radius, strongly support this claim. The ﬁrst point is that the rate of vanishing of π1∞ is rather related to higher (i.e. dimension n − 2) connectivity radii, which are less understood. The second difﬁculty is that these groups are not s.c.i. The simplest way to overcome it is to consider group extensions. For instance π1∞ (G × Z2 ) = 0, for any ﬁnitely presented group G; alternatively π1∞ (V n × R) = 0 for any contractible manifold V n (n 2). However this idea does not work because VG×Z2 is always linear. R e m a r k 7. One needs some extra arguments in order to extend the proof to all solvable Lie groups. However, it seems very likely that cocompact lattices in all connected Lie groups have linear rate of vanishing of π1∞ . 2. Proof of Theorem 1. For positive d, set Pd (G) for the simplicial complex deﬁned as follows: • its vertices are the elements of G, • the elements x1 ......xn of G span an n-simplex, if d(xi , xj ) d for all i, j (where d(., .) is the word metric). R e m a r k 8. If G is δ-hyperbolic then Pd (G) is contractible as soon as d > 4δ + 1 (see [8]).

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Louis Funar and Daniele Ettore Otera

arch. math.

Although Pd (G) is not contractible in general, one can prove that it is simply connected under a mild restriction. Let G = x1 , . . . , xn |R1 , . . . , Rp be a presentation of G and r denotes the maximum length among the relators Ri . Lemma 1. If 2d > r, then π1 (Pd (G)) = 0. P r o o f. Let l = [1, γ1 , γ2 , . . . , γn , 1] be a (simplicial) loop in Pd (G) based at the identity. Two successive vertices of l are at distance at most d. One can interpolate between two consecutive γj ’s a sequence of elements of G (of length at most d), consecutive ones being adjacent when viewed as elements of the Cayley graph (hence at distance one). The product of elements corresponding to these edges −1of length one of l is trivial in G. Therefore it is a product of conjugates of relators: gi Ri gi . The diameter of each Ri is at most r/2, and the assumption 2d > r, implies that each loop gi−1 Ri gi is contractible in Pd (G). This ends the proof. The natural group action of G on itself by left translations gives rise to an action on Pd (G). In particular, if G has no torsion then it acts freely on Pd (G) and Pd (G)/G = X is a compact simplicial complex with π1 (Pd (G)/G) = G. Proposition 1. The vanishing of π1∞ is a geometric property of torsion-free groups. P r o o f. One has to show that if the group H is quasi-isometric to G then π1∞ Pd (G) = 0 implies that π1∞ Pa (H ) = 0 for large enough a. Let f : H → G and g : G → H be (k, C)-quasi-isometries between G and H . Fix x0 ∈ Pa (H ) and f (x0 ) ∈ Pd (G) as base points. Lemma 2. If π1∞ Pd (G) = 0 then π1∞ PD (G) = 0 for D d. P r o o f. An edge in PD (G) corresponds to a path (of length uniformly bounded by D d +1) in Pd (G). Thus a loop l in PD (G) at distance at least R from a given point corresponds to a loop l in Pd (G) at distance at least R − D d − 1 from the same point. By assumption l will bound a 2-disk D 2 far away in Pd (G). Now the union of an edge [x1 xn ] in PD (G) and its corresponding path [x1 , x2 , . . . , xn ] in Pd (G) ⊂ PD (G) form the boundary of a 2-disk in PD (G), which is triangulated by using the triangles [x1 , xj , xn ]. Consider one such triangulated 2-disk for each edge of l and glue to the previously obtained D 2 to get a 2-disk in PD (G) bounding l and far away. By hypothesis for each r there exists N (r) > 0 such that every loop l in Pd (G) satisfying d(l, f (x0 )) > N (r) bounds a disk outside B(f (x0 ), r). This means that there exists a simplicial map ϕ : D 2 → Pd (G) − B(f (x0 ), r) such that ϕ(∂D 2 ) is the given loop l, when D 2 is suitably triangulated. A loop l = [x1 , x2 , . . . , xn , x1 ] in Pd (G), based at x1 , is the one-dimensional simplicial sub-complex with vertices xj and edges [xi xi+1 ], i = 1, n (with the convention n + 1 = 1).

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Set M(R) = kN (kR + kC + 3C) + 3C. We claim that: Lemma 3. Any loop l in Pa (H ) sitting outside the ball B(x0 , M(R)) bounds a 2-disk not intersecting B(x0 , R). P r o o f. Set l = [x1 , . . . , xn ]. Using the previous lemma one can assume that d is large enough such that d−C k > 1. As in lemma 1 one can add extra vertices between the consecutive ones such that d(xi , xi+1 ) ε holds, where kε + C = d. The image f (l) = [f (x1 ), . . . , f (xn ), f (x1 )] of the loop l has the property that d(f (xi ), f (xi+1 )) kε + C. Using d(x, gf (x)) C one obtains that d(gf (x), gf (y)) d(x, y) − 2C, which implies d(x, y) kd(f (x), f (y)) + 3C and thus: d(f (x), f (y))

d(x, y) − 3C , for all x, y ∈ Pa (H ). k

From this inequality one derives that: d(f (xi ), f (x0 ) (M(R) − 3C)/k = N (kR + kC + 3C) and thus the loop f (l) sits outside the ball B(f (x0 ), N (kR + kC + 3C)), and hence by assumption f (l) bounds a disk which does not intersect B(f (x0 ), kR + kC + 3C). Let y1 , . . . , yt be the vertices of the simplicial complex ϕ(D 2 ) bounded by the loop f (l). The vertices f (x1 ), . . . , f (xn ) are contained among the yj ’s. One can suppose that any triangle [yi , yj , ym ] of ϕ(D 2 ) has edge length at most d. Therefore we have: d(g(yj ), xi ) d(g(yj ), gf (xi )) + C kd(yj , f (xi )) + 2C k 2 ε + (k + 2)C.

This proves that xi , xj , g(ym ) span a simplex of Pa (H ) (for all i, j, m) whenever we choose a larger than k 2 ε + (k + 2)C. Moreover: d(x0 , g(yi )) d(gf (x0 ), g(yi )) − C

d(f (x0 ), yi ) − 3C − C R. k

Further there is a simplicial map ψ : ϕ(D 2 ) → Pa (H ) which sends f (xj ) into xj and all other vertices yk into the corresponding g(yk ). It is immediate now that ψϕ(D 2 ) is a simplicial sub-complex bounded by l, which has the required properties. This proves proposition 1. When G has torsion, one can construct a highly connected polyhedron with a free and cocompact G-action as follows (see [1]): D e f i n i t i o n 5. The colored Rips complex P (d, m, G) (for natural m) is the subcomplex of the m-fold join G ∗ G..... ∗ G consisting of those simplexes whose vertices are at distance at most d in G. Lemma 4. For m 3 and d large enough, G acts freely on the 2-skeleton of P (d, m, G) and π1 (P (d, m, G)) = 0.

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Louis Funar and Daniele Ettore Otera

arch. math.

P r o o f. Clearly G acts freely on the vertices of Pd (G), hence any non-trivial g ∈ G ﬁxing a simplex has to permute its vertices. Adding m 3 colors prevents therefore the action from having ﬁxed simplexes of dimension less than 3. Now using the proof of Lemma 1 one obtains also the simple connectivity. The Theorem 1 follows now from the proof of Proposition 1, suitably adapted to the 2-skeleton of P (d, m, G). R e m a r k 9. The same technique shows that the higher connectivity at inﬁnity is also a quasi-isometry invariant of groups. We have then: Corollary 2. The equivalence class of VX (r) is a quasi-isometry invariant of X. P r o o f. The result is implied by Theorem 1 and Lemma 3.

3. Uniform lattices in Lie groups. Proposition 2. Uniform lattices in (non-compact) semi-simple Lie groups have linear rate of vanishing of π1∞ . P r o o f. We will denote below by dX the distance function and by BX the respective metric balls for the space X. Let K be the maximal compact subgroup of the simple Lie group G and G/K the associated symmetric space. It is well-known that the Killing metric on G/K is non-positively curved, and hence the metric balls are diffeomorphic to standard balls, by the Hadamard theorem. If G is not SL(2, R) then K is different from S 1 and therefore it has ﬁnite fundamental is compact. The Iwasawa decomposition group. In particular the universal covering K G = KAN yields a canonical diffeomorphism G → K × G/K. Furthermore we have →K × G/K. Large balls in G can be therefore an induced canonical quasi-isometry G and metric balls in G/K, as follows: compared with products of the (compact) K × BG/K (r − a) ⊂ BG K (r) × BG/K (a + r) ⊂ BG ⊂K (2a + r), for r large enough, which implies our claim. The case of VSL(2, R) is quite similar, and a consequence of the (well-known) fact that 2 SL(2, R) and H × R are canonically quasi-isometric. We will sketch a proof below. Let R) geometry. A Riemannian metric on a manius outline the construction of the SL(2, fold allows us to construct canonically a Riemannian metric on its tangent bundle, usually called the Sasaki metric. In particular one considers the restriction of the Sasaki metric to the unit tangent bundle UH 2 of the hyperbolic plane. Further there exists a natural diffeomorphism between UH 2 and PSL(2, R), which gives PSL(2, R) a Riemannian metric, and R). This is the Riemannian hence induces a metric on its universal covering, namely SL(2,

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R). Observe that SL(2, R) and H 2 × R are two structure describing the geometry of SL(2, metric structures on the same manifold, and both are Riemannian ﬁbrations over H 2 (the former being metrically non-trivial while the later is trivial). It is clear now that the identity map between the manifolds UH 2 and H 2 × S 1 is a quasiR) and H 2 × R. This implies that there isometry, lifting to a quasi-isometry between SL(2, are two constants a > 0, b such that: 1 d 2 (x, y) − b dSL(2, R) (x, y) a H ×R adH 2 ×R (x, y) + b, for all x, y ∈ H 2 × R, holds true. In particular we have the following inclusions between the respective metric balls: r 2 ⊂ BSL(2, BH 2 ×R R) (r) ⊂ BH 2 ×R (cr) ⊂ BSL(2, R) (c r), c for r large enough and c > 0. The claim follows.

R e m a r k 10. The same argument shows that the acyclicity radius for semisimple Lie groups is linear (see [11], Section 4). The way to prove the claim for nilpotent and solvable groups consists in the large scale comparison with some other metrics, whose balls are known to be diffeomorphic to standard balls. While locally the Riemannian geometry of a nilpotent Lie group is Euclidean, globally it is similar to the Carnot-Caratheodory non-isotropic geometry. Proposition 3. If G is a torsion-free nilpotent group then VG is linear. P r o o f. It is known (see [13]) that G is a cocompact lattice in a real simply connected nilpotent Lie group GR , called the Malcev completion of G. We have also a nice characterization of the metric balls in real, nilpotent Lie groups given by Karidi (see [12]), as follows. Since GR is diffeomorphic to Rn it makes sense to talk about parallelepipeds with respect to the usual Euclidean structure on Rn . Next, there exists some constant a > 0 (depending on the group GR and on the left invariant Riemannian structure chosen, but not on the radius r) such that the radius r-balls BGR (r) are sandwiched between two parallelepipeds, which are homothetic at ratio a, so: Pr ⊂ BGR (r) ⊂ aPr ⊂ BGR (ar), for any r 1. This implies that we can take VGR (r) ∼ ar, and hence VG is linear.

R e m a r k 11. Metric balls in solvable Lie groups are quasi-isometric with those of discrete solvgroups, and so they are highly concave (see [7]): there exist pairs of points at distance c, sitting on the sphere of radius r, which cannot be connected by a path within the ball of radius r, unless its length is at least r 0.9 . Further it can be shown that there are arbitrarily large metric balls which are not simply connected. Nevertheless we will prove that metric balls contain large slices of hyperbolic balls.

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Louis Funar and Daniele Ettore Otera

arch. math.

Proposition 4. Cocompact lattices in solvable stabilizers of horospheres in product of symmetric spaces of rank at least two, or generic horospheres in products of rank one symmetric spaces have linear VG . P r o o f. We give the proof for our favorite solvable group, namely the 3-dimensional group Sol. It is well-known that Sol is isometric to a generic horosphere H in the product H 2 × H 2 of two hyperbolic planes. Generic means here that the horosphere is associated to a geodesic ray which is neither vertical nor horizontal. The argument in ([11] 3.D.), or its generalization from [6], shows that such horospheres H are undistorted in the ambient space i.e. there exists a 1, such that 1 d 2 2 (x, y) dH (x, y) adH 2 ×H 2 (x, y), for all x, y ∈ H, a H ×H holds true. Here dH 2 ×H 2 and dH denote the distance functions in H 2 × H 2 and H, respectively. In particular we have the following inclusions between the respective metric balls: r ⊂ BSol (r) ⊂ H ∩ BH 2 ×H 2 (ar) ⊂ BSol (a 2 r). H ∩ BH 2 ×H 2 a Since the horoballs in H 2 × H 2 are convex it follows that the intersections H ∩ BH 2 ×H 2 (r) are diffeomorphic to standard balls. This proves that VSol is linear. The linearity result extends without modiﬁcations to lattices in solvable stabilizers of generic horospheres in symmetric spaces of rank at least 2 (see [6]). R e m a r k 12. It is known that ﬁnitely presented solvable groups are either simply connected at inﬁnity or are of a very special form, as described in [14]. On the other hand it is a classical result that any simply connected solvable Lie group is diffeomorphic to the Euclidean space. It would be interesting to know whether a simply connected solvable Lie group can be isometrically embedded as a horosphere in a symmetric space. R e m a r k 13. Notice that horospheres in hyperbolic spaces (and hence non-generic horospheres in products of hyperbolic spaces) have exponential distortion, namely dH n (x, y) ∼ log dHn−1 (x, y), for x, y ∈ Hn−1 . This highly contrast with the higher rank and/or generic case. This ends the Proof of Theorem 2. R e m a r k 14. One might notice a few similarities between VG and the isodiametric function considered by Gersten (see [11]). Corollary 3. The rate of vanishing of π1∞ is linear for the fundamental groups of geometric 3-manifolds. P r o o f. There are eight geometries in the Thurston classiﬁcation (see [16]): the sphere R), Nil and Sol. S 3 , S 2 × R, the Euclidean E 3 , the hyperbolic 3-space H 3 , H 2 × R, SL(2, Manifolds covered by S 3 have ﬁnite fundamental groups. Further the compact manifolds

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without boundary covered by S 2 × R are the two S 2 bundles over S 1 , RP2 × S 1 or the connected sum RP3 RP3 , and the claim can be checked easily. As we already observed, this is the also the case for the Euclidean and hyperbolic geometries. The same holds for the product H 2 × R, in which case metric balls are diffeomorphic to standard balls. The remaining cases are covered by Theorem 2. A c k n o w l e d g e m e n t s. The authors are indebted to C. Drut¸u, P. Pansu and F. Paulin for helpful conversations and to the referee for the useful comments improving the exposition. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

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L. Funar Universit´e Grenoble I Institut Fourier UMR 5582 BP 74 38402 Saint-Martin-d’H`eres Cedex France [email protected]

D. E. Otera Universit´e Paris-Sud Departement de Math´ematiques Bˆatiment 425 F-91405 Orsay Cedex France [email protected]